Fusion bonding, or "welding" of thermoplastic composite material, such as polyimide impregnated graphite, is an emerging technology of great promise in the aerospace field for reducing the cost of fastening parts, such as wing spars, ribs and wing skins, together to make large assemblies, such as a wing box. A large percentage of the cost of the final product that is fastened together by conventional fasteners such as rivers, lock bolts, etc., is in the laborious process of drilling holes, installing fasteners and securing the fastener, usually one at a time.
An apparatus and method for performing welding of thermoplastic composite material is described in U.S. patent application No. 08/352,991 filed on Dec. 9, 1994, by John Mittleider and entitled "Thermoplastic Welding", the disclosure of which is incorporated herein by reference. The method described in the Mittleider application uses a conductive foraminous susceptor at the interface between two parts to be welded together. Eddy currents in the susceptor, induced by an alternating magnetic field generated by an induction coil, heat the susceptor by resistive heating and raise the temperature of the thermoplastic in the faying surfaces of the two parts in contact with the susceptor to the thermoplastic melt point. Pressure is applied to squeeze the two parts together, pressing the melted thermoplastic through and around the interstices of the susceptor and promoting molecular diffusion of the faying surfaces to form a bond region that is continuous and uniform from one part to the other with no discernible junction between the two parts, other than the embedded susceptor.
The DuPont Avamid KIII-B polyimide resin used in the parts described in the aforesaid Mittleider application melts in a range of T.sub.m =620.degree..+-.20.degree. F. The temperature of the melt pool in the interface between the two parts is a sensitive parameter affecting the quality of the bond. Temperatures above the optimal range can cause porosity in the bond region which could weaken the weld, and could also cause delamination and/or sagging of the adjacent structures because of excessive heating of the thermoplastic structure outside of the bond region. Temperatures below the optimal range can prevent the formation of a well "healed" fusion bond. The melt pool in the interface is hidden under the overlying graphite reinforced plastic, ruling out use of infrared radiation sensors.
It is possible to embed thermocouples within material or in an interface between two parts and obtain accurate temperature signals. However, the thermocouples are useful only for sensing the temperature at their position and they cannot be moved to obtain a continuous readout at the point of interest, for example, at the melt pool in a thermoplastic weld joint. The information obtained from use of thermocouples in a temperature feed-back system is thus intermittent and the conditions between thermocouples must be extrapolated from the data obtained from the upstream thermocouples. In a continuous process control system, intermittent data is be less desirable than continuous data.
Thus, the art would be greatly advanced by a continuous remote temperature sensor for thermoplastic welding and other remote temperature sensing applications which can accurately indicate the temperature within a body of material that is not suitable for the application of other conventional temperature sensors.